This invention relates to an apparatus for controlling a power converter, which is adapted to control, in accordance with the PWM system, a power converter for system interconnection interposed between the d.c. power source and the a.c. system to adjust interchange power therebetween.
Power converters for system interconnection of this kind may have an inverter operating mode used in supplying power from the d.c. power source to the a.c. system and a rectifier operating mode used in supplying power from the a.c. system to the d.c. power in a manner opposite to the above.
When it is assumed that a voltage on the a.c. side of a power converter is V.sub.in, a voltage of the a.c. system is V.sub.sy, a reactance between the d.c. power source and the a.c. system is X, and a phase difference angle between the voltage V.sub.in and the voltage V.sub.sy is .phi. (which is positive when the phase of the voltage V.sub.in leads the phase of the voltage V.sub.sy), effective or active power P and reactive power Q processed in the power converter are respectively expressed as follows: EQU P={(V.sub.in .multidot.V.sub.sy)/X} sin .phi. (1) EQU Q=(V.sub.in .multidot.V.sub.sy .multidot.cos .phi.-V.sub.sy .sup.2)/X(2).
Where the power converter is operated in the rectifier operating mode, the amplitude of voltage V.sub.in is smaller than that of voltage V.sub.sy and the phase of voltage V.sub.in lags that of voltage V.sub.sy. Thus, the power converter dissipates lagging reactive power Q and transmits effective power P from the a.c. system to the d.c. power source. Moreover, as apparent from the above equations (1) and (2), even if the amplitude of voltage V.sub.in is smaller than that of voltage V.sub.sy, as long as the phase of voltage V.sub.in leads that of voltage V.sub.sy, the power Converter the dissipates lagging reactive power Q and transmits the effective power P from the d.c. power source to the a.c. system.
When the effective value of the voltage V.sub.in satisfies the condition expressed by the following equation with respect to the effective value of the voltage V.sub.sy, EQU V.sub.in V.sub.sy /COS .phi. (3)
the power converter operates as a capacitor and transmits effective power from the a.c. system to the d.c. power source. When the phase difference angle .phi. is positive as apparent from the above equations (1) and (2) even in the case where the above equation (3) is satisfied, the power converter operates as a capacitor and transmits the effective power from the d.c. power source to the a.c. system.
When it is assumed that the voltage on the d.c. side of the power converter, i.e., the voltage of the d.c. power source is V.sub.dc, the modulation factor in the PWM Control of the power converter is M.sub.f (0.ltoreq.M.sub.f 1), and k is a constant, the voltage V.sub.in on the a.c. side of the power converter is expressed as follows: EQU V.sub.in =k.multidot.V.sub.dc .multidot.M.sub.f ( 4).
Substitution of the equation (4) into the equations (1) and (2) respectively gives: EQU P=V.sub.sy (K.multidot.M.sub.f .multidot.V.sub.dc /X) sin .phi.(5), and EQU Q={V.sub.sy (K.multidot.M.sub.f .multidot.V.sub.dc .multidot.cos .phi.)-V.sub.sy .sup.2 }/X (6).
As is apparent from the above equations (5) and (6), by controlling the modulation factor M.sub.f and/or the phase difference angle .phi. by the power converter, the effective power P and the reactive power Q can be adjusted.
Meanwhile, the three phases are not necessarily balanced with each other in an ordinary a.c. system voltage. There are many instances where three phases are imbalanced, for example, in a form such that amplitudes are different from each other in the respective R, S and T phases. Even in the case where the a.c. system voltage is imbalanced as stated above, the above-described power converter can interchange the effective and reactive powers between the a.c. system and the d.c. power source. However, because the three phases of the a.c. system voltage are imbalanced, the effective and reactive powers with three phases being as a lump under conditions where amplitudes of the output a.c. voltage are different from each other in the three phases take predetermined values, respectively. In this case, when the degree of imbalance becomes large, the output current of the converter of a specified phase will increase and therefore current over an allowed current will flow in controllable rectifier elements at the specified phase. As a result, it becomes impossible to continue the operation of the converter.
In addition to the above-described inconvenience, there is another inconvenience described below. Since there essentially exists unevenness in the characteristics of respective controllable rectifier elements even if the energization periods of respective controllable rectifier elements of the converter are determined, energization is not necessarily conducted as required by a designated energization period, so that there may be produced unevennesses between energization periods of respective controllable rectifier elements. Such unevennesses between energization periods of respective controllable rectifier elements causes the current on the a.c. side of the converter to produce d.c. components. The d.c. components mentioned above cause d.c. polarized magnetization in the interconnecting transformer ordinarily provided between the converter and the a.c. system, thus allowing the interconnecting transformer to over-heat, or otherwise extremely distorting an output current of the converter. When the occasion demands, there may occur a circumstance where a large peak current based on strain exceeds an allowed current value of the controllable rectifier element. Such circumstances make the operation of the converter impossible.